Marcus flinched at the sharp contact with his shin, but was still able to swing that leg around behind John "s and trip him up.  He let go of the other's shirt so he was free to fall but was still clipped in the jaw by John's elbow. 
Marcus was no stranger to close quarters hand to hand, it having been a crucial part of his training as well as his day to day survival down below.  But this fight had caught him off guard.  And worse then that, it was personal and emotional.  He wasn't being rational or logical here, wasn't trying to take John down as quickly as possible anymore.  He was just angry.